There are two basic types of multi-view display device. One type is display devices for simultaneously presenting different images to different viewers at different spatial locations. For example, one viewer can be in the driver's seat of a vehicle and another can be in the passenger's seat. The driver can be displayed driver-related content such as satellite navigation content, and the passenger can be displayed entertainment content. There may be more than two views, for multiple viewers.
Another type is autostereoscopic displays for displaying the views for different eyes to different spatial locations. There may be two views, for a single viewer at a single position, but there may be many more views (e.g. 9 or 15) so that multiple viewers can be in the field of view and/or so that a viewer can move relative to the display to experience a look around effect.
The principles behind the generation and spatial separation of the different views are the same in these devices. Essentially, different 2D content is projected to different spatial locations simultaneously. The only difference is that the angular separation of views is typically less for the autostereoscopic display (approximately 2 degrees) than for multi-view displays, where both eyes of a viewer receive the same image. Typically, the different spatial locations extend along a horizontal line, since viewers' eyes are generally at the same vertical height, but at different horizontal positions with respect to the display. For the purposes of explanation, the invention will be described with reference to autostereoscopic display devices.
A known autostereoscopic display device comprises a two-dimensional liquid crystal display panel having a row and column array of display pixels acting as a spatial light modulator to produce the display. An array of elongate lenticular elements extending parallel to one another overlies the display pixel array, and the display pixels are observed through these lenticular elements. The lenticular elements are provided as a sheet of elements, each of which comprises an elongate semi-cylindrical lens element. The lenticular elements extend in the column direction of the display panel, with each lenticular element overlying a respective group of two or more adjacent columns of display pixels.
In an arrangement in which, for example, each lenticular element is associated with two columns of display pixels, the display pixels in each column provide a vertical slice of a respective two-dimensional sub-image. The lenticular sheet directs these two slices and corresponding slices from the display pixel columns associated with the other lenticular elements, to the left and right eyes of a user positioned in front of the sheet, so that the user observes a single stereoscopic image. The sheet of lenticular elements thus provides a light output directing function.
In other arrangements, each lenticular element is associated with a group of, say, four or more adjacent display pixels in the row direction. Corresponding columns of display pixels in each group are arranged appropriately to provide a vertical slice from a respective two-dimensional sub-image. As a user's head is moved from left to right a series of successive, different, stereoscopic views are perceived creating, for example, a look-around impression.
The above-described device provides an effective three-dimensional display. However, it will be appreciated that, in order to provide stereoscopic views, there is a necessary sacrifice in the horizontal resolution of the device (the same applies to the resolution of the different images in a multi-view 2D system). This sacrifice in resolution is unacceptable for certain applications, such as the display of small text characters for viewing from short distances or graphics applications needing a high resolution. For this reason, it has been proposed to provide an autostereoscopic display device that is switchable between a two-dimensional (2D) mode and a three-dimensional (3D stereoscopic) mode. Such a device is described in U.S. Pat. No. 6,069,650, the entirety of which is incorporated herein by reference. In this device, different groups of pixels, forming one or more stereoscopic pairs, are seen by respective eyes of a viewer through the lenticular elements. The lenticular elements include electro-optic material having a refractive index that is switchable in order to enable removal of the refracting effect of the lenticular elements.
In the two-dimensional mode, the lenticular elements of the switchable device operate in a “pass through” mode, i.e. they act in much the same way as would a flat sheet of optically transparent material. The resulting display has a high resolution, equal to the native resolution of the display panel, which is suitable for the display of small text characters from short viewing distances. The two-dimensional display mode cannot, of course, provide a stereoscopic image.
In the three-dimensional mode, the lenticular elements of the switchable device provide a light output directing function, as described above. The resulting display is capable of providing stereoscopic images, but also suffers the inevitable resolution loss mentioned above.
In order to provide switchable display modes, the lenticular elements of the switchable device use an electro-optic material, such as a liquid crystal material, having a refractive index that is switchable between two different values for polarized light. The device is then switched between the modes by applying an appropriate electrical potential to electrode layers provided above and below the lenticular elements. The electrical potential alters the refractive index of the lenticular elements in relation to that of an adjacent optically transparent layer. Alternatively, the adjacent optically transparent layer may be formed of the electro-optic material, with the same result that the refractive index of the lenticular elements in relation to the optically transparent layer is altered.
Problems have been experienced, however, with unwanted display artefacts in the 2D mode of a switchable multiview/single view display when viewed at oblique angles. At such angles, a visible structure is present in the display output that appears to be related to the structure of the lenticular sheet, while no such structure is visible when viewing orthogonal to the plane of the display panel and the lenticular array.
A number of different approaches have been suggested for addressing this problem. For example, WO2007/099488 discloses the use of a birefringent (non switchable) replica structure adjacent the switchable birefringent lens structure. By forming the optically transparent layer of the lenticular means from birefringent material, considerable improvement in the reduction of the aforementioned unwanted display artefacts in 2D mode displays at oblique angles can be achieved. However, the additional complexity of a birefringent replica structure is needed. There are also other solutions to improve the image quality at lateral viewing angles, but these solutions all introduce additional complexity to the display.